1. Field of the Invention
The present invention relates to automatic focusing devices conducting a focusing operation in accordance with imaging signals obtained from an image sensor, such as CMOS sensor, and provided in imaging devices such as a digital camera or a digital video camera.
2. Description of Related Art
Conventionally, in a digital camera provided with a CMOS sensor as an imaging device, while a frame period V is repeated as shown in FIG. 4, an exposure is performed at every horizontal scanning line sequentially from the highest horizontal scanning line La to the lowest horizontal scanning line Lb of an imaging picture obtained by the CMOS sensor. As a result, imaging signals for one frame are obtained by the CMOS sensor. This exposure system is called a rolling shutter system.
In the imaging operation described above, a focus lens is driven from the infinity position to the closest position, and autofocus operation to focus on the CMOS sensor is conducted based on the imaging signals obtained in this process. In the autofocus operation, a part of the imaging picture taken by the CMOS sensor is set as a focusing area. For example, in the case where a person's face is included in an imaging picture, the area including the face is automatically set as a focusing area. And then an autofocus evaluation value (for example, an integration value of high-frequency components) of the focusing area of every frame is calculated, and the position of the focus lens is adjusted so that the autofocus evaluation value becomes maximum.
In a digital camera having an image sensor of the rolling shutter system such as the CMOS sensor, the exposure periods are staggered in time from one horizontal scanning line to another as shown in FIG. 4. Therefore, a period D for driving the focus lens overlaps the exposure period.
In this case, since the autofocus evaluation value obtained by the exposure does not correspond to a certain focus lens position, conventionally, a given focus lens position such as a position at the time of exposure for the center line of the screen is regarded as a focus lens position corresponding to the evaluation value of the frame. However, this tentative focus lens position does not correspond to the autofocus evaluation value of the frame accurately, and therefore there has been a problem of low accuracy of peak detection in the autofocus operation.
Therefore, it has been proposed to increase the accuracy of the autofocus operation by reducing the driving amount of the focus lens for one frame so that the exposure period and the focus lens driving period do not overlap. However, in this method, in order for the exposure period and the focus lens driving period not to overlap, the driving amount of the focus lens for one frame must be reduced. Therefore, there has been a problem of taking a long time to complete focusing.